build: Detect host and sources version

* We need the Xen sources when building xcl, and proper versioning
needs to be checked in order to avoid building errors. Often libxc
and libxl change its API and we need to at least prevent cross
versioning of these libraries.
docs: Notes on generating header for XEN_ROOT
xcl: PVH is only supported for >= Xen 4.4.0

PVH support broke cosmos for Xen 4.3.0 and below. Make the
necessary code build only on Xen 4.4.0 and above.